The helix-loop-helix (HLH) factor Id3 has emerged as an important regulator of vascular smooth muscle cell (VSMC) growth. Id3 is thought to promote proliferation by inhibiting the expression of the cyclin-dependent kinase inhibitor (cdki) P21cip1, which has been implicated as an important inhibitor of VSMC proliferation and vascular lesion formation. However recent data suggest additional mechanisms of Id3-induced growth regulation. Id3 is regulated by cyclin-dependent kinase 2 (cdk2) phosphorylation. Phosphorylation of transcription factors represents a major means by which signal transduction and gene expression are integrated. Cdk2 phosphorylation of Id3 on serine 5 enhances Id3 antagonism of a MyoD/E12 complex on EMSA and inhibited S-phase entry in rat embryonic fibroblasts (REF). Our preliminary data in VSMC confirm the importance of this phosphorylation site on Id3, but suggest that in VSMC serine 5 phosphorylation of IdS regulates growth through other mechanisms. We provide the first evidence that phosphorylation provides a regulatory switch controlling the effects of Id3 on regulation of p21Clp1 promoter activation in VSMC. In addition, we provide the first evidence that Id3 functions as a regulator of G1-S progression in VSMC through both p21dp1-dependent and p21cip1- independent mechanisms. Accordingly, we hypothesize that cdk2 phosphorylation of IdS on serine 5 is a key regulatory event that determines the unique functional role of IdS as either an inducer or inhibitor of p21cip1 gene expression leading to VSMC proliferation. We hypothesize that nonphosphorylated Id3 dimerizes with a factor that acts as a repressors of p21cip1 gene transcription such that an increase in nonphosphorylated IdS results in an increase in the expression of p21cip. When Id3 is phosphorylated on serine 5, it no longer binds this repressor and instead interacts with E47 and inhibits E47-mediated transcription of p21cip1 allowing S-phase entry. We further hypothesize that cdk2 phosphorylation of Id3 also regulates Id3-induced effects on other important regulators of G1-S progression such as cyclin D, cdk2 and Rb. To test these hypotheses we propose: to determine the effects of IdS and cdk2-phosphorylation of IdS at serine 5 on cell cycle progression and growth in VSMC; to identify the molecular mechanisms whereby serine 5 phosphorylation regulates IdS-induced effects on p21cip1 expression and VSMC cell cycle progression; and to extend in vitro findings into an in vivo model to study the effects of Id3 on vascular lesion formation.